A Polish-led multinational force has taken over control of part of central Iraq, in a move by the United States to relieve some of the burden on its own troops.

US marines handed over to the 21-nation force in a ceremony in the ancient city of Babylon, making Poland the third country after the US and UK to accept responsibility as an occupying force.

The 9,000-strong force will operate in an area south of the capital Baghdad which includes the holy city of Najaf, where a car bomb last Friday killed the leading Shia cleric, Ayatollah Muhammad Baqr al-Hakim.